Sebastian Vettel will join "a winning team" for 2021 or quit the sport, according to his former boss Dr Helmut Marko.
Speculation about the quadruple world champion's future is rife after Ferrari announced that he will be replaced by Carlos Sainz next year.
"I have spoken to Sebastian and of course we went through the options at Red Bull, but unfortunately there are none," Marko, who brought Vettel into the sport via Red Bull's driver program, told Sky Germany.
"When I spoke to him, the message was that if he can join a winning team, he continues. Otherwise not."
Indeed, after McLaren chiefs revealed that no talks with 32-year-old Vettel took place, it is believed Renault is also not an option for the German.
However, there are now rumours that Aston Martin may be a possibility.
F1 business journalist Christian Sylt reported in the British publication This Is Money that documents show Racing Point owner Lawrence Stroll is paving the way for Aston Martin to invest up to $243m in the re-branded team.
"Of course I think it is possible that Vettel goes to an Aston Martin team that will have Mercedes engines," said RTL commentator Florian Konig.
Earlier, it was suggested Vettel was eyeing only the works Mercedes team, but Ralf Schumacher thinks there is "a lot under discussion" going on at present.
"There is the possibility that Toto Wolff, in combination with Lawrence Stroll, can offer him something," the former F1 driver said.
Schumacher thinks it is possible that Wolff and Stroll may even "buy Mercedes".
Konig continued: "I think there will be intensive work behind the scenes to ensure that Sebastian will still be in Formula 1 in 2021."
Some think Mercedes parent Daimler will be pushing hard to potentially pair Vettel with Lewis Hamilton from 2021.
"If I interpret correctly, the decision rests with Mercedes," Marko said. "I think Hamilton against Sebastian would mobilise a very, very large number of spectators."
Marko also thinks Vettel could simply retire.
"It would be a shame for the sport, but for him it would be the best solution if there is no winning team available," he said.
Vettel's friend and mentor, the former F1 supremo Bernie Ecclestone, thinks it really is possible that Mercedes will pair Hamilton and Vettel together.
"The two get along. I don't see any ego problems and two outstanding drivers like them would be a super team," he told RTL.
"I can't imagine that Hamilton would worry about Sebastian and I know that Vettel would also enjoy the challenge of fighting Lewis in the same car."
